Sharded Servers for MongoDB
I
Iyobo Eki
This is necessary for serious apps
I
Ilja Daderko
+1 Not just due to price, but as an option to scale beyond single server limits. It is often more beneficial to have more shards with lower specs for high throughput, rather than one big db.
I believe sharding will be easiest to achieve / start with for mongodb and redis, as both of them support it.